Comment 9 for bug 1827452

Update again.
The kernel maintainers have been helpful but prefer NULL checking to be done in the drivers, not in the kernel. I've explored further with some test builds and it turns out the NULL is actually on the dev config interfaces (usb.c:281 current line). During initialization the Logitech C920 sometimes unplugs/deactivates itself. Apparently many Logitech models do this if you search the issue - Windows included. Shame to have such a great quality camera paired with such subpar firmware.

Anyway I'm trying fix uvc_driver.c to verify devices are still plugged in before calling usb_ifnum_to_if. It won't fix webcams or devices that unplug themselves (or get unplugged) during initialization but it will protect the USB subsystem so you can plug it back in again.